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Java 8流分组以对象作为键的对象字段

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我有以下类的结构（给出的例子是没有意义的，我只是为了问题目的而做的）：

public class ManufacturerDto {
    private String name;
    private Boolean withConfirmation;

    //getters and setters omitted for brevity
}

public class CarDto {

    private ManufacturerDto manufacturer;

    //getters and setters omitted for brevity
}

public abstract class VehicleDto {

    private list<CarDto> cars = new ArrayList<>();

    //getters and setters omitted for brevity
}

public class EuropeanMarketCarDto extends VehicleDto {

    public Map<String, List<CarDto>> getCarManufacturerToCarsMap() {

       return this.getCarDtos()
                .stream()
                .collect(Collectors.groupingBy(e -> e.manufacturer().getName()));

        //getters and setters omitted for brevity
    }
}

在给定的场景中， groupingBy 几乎可以返回我想要的所有内容，但是我的密钥有问题 . 在 getCarManufacturerToCarsMap() 方法中，我希望将 manufacturer 对象作为键，但我甚至不确定是否可以使用 groupingBy . 我该怎么做才能返回 Map<Manufacturer, List<CarDto>> 的 Map ？

java java-stream

2 回答

  • 2

    如果您确实希望避免将 Map 密钥设为 String ，即 Map<String, List<CarDto>> 而是拥有 Map<Manufacturer, List<CarDto>> ，则可按以下方式分组：

    this.getCarDtos()
      .stream()
      .collect(Collectors.groupingBy(e -> e.getManufacturer()));

    请注意，这将要求您在 Manufacturer 类中重写 equals 和 hashcode 方法，以定义两个给定对象具有相同名称的相等对象 .

    回复于
  • 2

    你可以尝试这样的事情：

    Map<Manufacturer, List<Car>> listing = cars.stream().collect(Collectors.groupingBy(Car::getManufacturer));

    因此，这将由它的制造商对所有汽车进行分组，输出将是这样的：

    TOYOTA = [AURIS的AURIS params，AVIA的AVIA params]，MERCEDES = [S500的S500 params ...]

    EDIT 2

    public class Main {

    List<Car> listOfCars = new ArrayList<>();


    public static void main(String[] args) {
    List<Car> listOfCars = new ArrayList<>();
    listOfCars.add(new Car(new Manufacturer("Toyota"), "Toyota A"));
    listOfCars.add(new Car(new Manufacturer("Toyota"), "Toyota B"));
    listOfCars.add(new Car(new Manufacturer("Jeep"), "Jeep A"));
    listOfCars.add(new Car(new Manufacturer("Mercedes"), "Mercedes C"));
    listOfCars.add(new Car(new Manufacturer("Toyota"), "Toyota C"));
    listOfCars.add(new Car(new Manufacturer("Audi"), "Audi A"));
    listOfCars.add(new Car(new Manufacturer("Toyota"), "Toyota C"));



 Map<String,List<Car>> listOfCarsByManufacturers = listOfCars.stream().collect(Collectors.groupingBy(d->d.getManufacturer().getName()));

    System.out.println(listOfCarsByManufacturers);
    }




}

    public class Car {

    private Manufacturer manufacturer;
    private String nameOfTheCar;

    public Car() {
    }

    public Car(Manufacturer manufacturer, String nameOfTheCar) {
        this.manufacturer = manufacturer;
        this.nameOfTheCar = nameOfTheCar;
    }

    public Manufacturer getManufacturer() {
        return manufacturer;
    }

    public String getNameOfTheCar() {
        return nameOfTheCar;
    }

    @Override
    public String toString() {
        return "Manufacturer: " + getManufacturer() + ", name: " + getNameOfTheCar();
    }

}

public class Manufacturer {

    private String name;

    public Manufacturer() {
    }

    public Manufacturer(String name) {
        this.name = name;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    @Override
    public String toString() {
        return getName();
    }


}

    EDIT 3:

    Map<String,List<Car>> listOfCarsByManufacturers = listOfCars.stream().collect(Collectors.groupingBy(d->d.getManufacturer().getName()));
System.out.println(listOfCarsByManufacturers);
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